1. Field of the Invention
The present invention relates to a unit of arrayed light emitting diodes. More particularly, the present invention relates to a unit of a large number of light emitting diode arrays on a substrate to be utilized in an optical printer head and the like.
2. Description of the Prior Art
As an optical printer using light emitting diode arrays as a light source, U.S. Pat. No. 3,850,517 to Joseph F. Stephany, entitled "High Speed Printout System" is known. Further, as light emitting diode arrays to be used for such an optical printer, U.S. Pat. No. 3,967,893 to Edward John Majewicz, entitled "Light Emitting Device" and Japanese patent Laying-Open Gazette No. 125878/1983 "Light Emitting Device Fixing Substrate" invented by Takeshi Mizutani et al are known.
The "Light Emitting Device" of U.S. Pat. No. 3,967,893 discloses that a drive circuit for enabling a large number of light emitting diode arrays on a substrate is disposed outside the substrate where the light emitting diode arrays are placed.
FIG. 1A is a perspective view showing an appearance of an example of a conventional light emitting device using a "light emitting device fixing substrate" in Japanese patent Laying-Open No. 125878/1983, and FIG. 1B is a view showing an arrangement of the light emitting diode arrays shown in FIG. 1A.
Referring to FIG. 1A, a light emitting device to be used in a conventional optical printer head will be described. The light emitting device 1 is constructed by arranging on a substrate 2 a large number of light emitting diode arrays 4 having light emitting areas 3. Each of the light emitting diode array 4 is connected to a fine pattern 6 formed adjacent to each light emitting diode by metallic fine wire.
If an optical printer head comprises the light emitting device 1 thus formed and characters and the like are printed on printing paper of the A4 size using this optical printer head, the effective printing length needs be as long as 214 mm, and if characters are printed on printing paper of the A2 size, the effective printing length needs be as long as 430 mm. However, since it is impossible to manufacture a light emitting diode array having such length, in order to meet the condition of such effective length, it is necessary to arrange a large number of light emitting diode arrays 4 on the substrate 2.
In order to improve the resolution of the printed letters or picture, it is necessary to make small and equal the dot pitches of printing, that is, the intervals of the light emitting areas 3. To obtain a resolution of 8 to 25 dots for 1 mm, the light emitting areas 3 on the surfaces of the light emitting diode arrays 4 need be arranged with a pitch of 0.125 to 0.04 mm. Such pitch must be invariably equal not only on the surfaces of the light emitting diode arrays 4 but also in the connection points of the light emitting diode arrays 4 so as not to deteriorate the quality of printing of an optical printer. Further, if a disorder occurs in the arrangement with such small intervals of a large number of light emitting areas 3, or light emitting diode arrays 4 on the substrate 2, the linearity in printing will be disturbed, causing an inferior quality of printing.
Therefore, in order to maintain a good quality of printing, it is necessary, as shown in FIG. 1B, to decrease a deviation T from the center line (a) over the whole length L to .+-.100 .mu.m or less irrespective of the effective length, and to decrease to +10 .mu.m or less the maximum distance (deviation) t between the center lines of the respective two adjacent light emitting diode arrays.
To arrange light emitting diode arrays 4 on the substrate at the above stated dot pitches with such precision, it is necessary to use a magnifier. However, the range of vision of a magnifier utilized for keeping the above state precision is 15 mm at most and if an optical printer head having an effective printing length of 214 mm to 430 mm as described above is utilized, the light emitting diode arrays 4 cannot be arranged in a straight line, causing sinuosity as a whole. Further, even if a pattern identifying device is utilized to automatically arrange light emitting diode arrays 4 on the substrate 2, it is extremely difficult to arrange a large number of light emitting diode arrays 4 on the substrate 2 by checking the linearity along the total length with predetermined dot pitches being maintained.
In consequence, a method for arranging light emitting diode arrays 4 in grooves 7 formed in the substrate 2 shown in FIG. 1A may be considered. However, it is also extremely difficult to form elongate grooves of 1 to 2 mm in width almost coincident with the light emitting diode arrays 4. More specifically, if elongate grooves 7 are formed in the substrate 2 by a mechanical method, the linearity cannot be maintained with good precision and if the grooves are formed by etching or other similar method, it is difficult to make plane the bottom surfaces of the grooves 7, causing inconvenience such as inclination of the surfaces of the light emitting diode arrays 4.